1. Field of the Invention
The present invention relates to a method of deciding the quality of a resin workpiece in a compression-molding process and to a method of controlling the pressure of the working fluid on a compression-molding press.
2. Description of the Prior Art
The front panel, body panel and the like of automobiles are formed by compression-molding a SMC (sheet molding compound) by a press. The SMC is a composite material consisting of base materials including a thermosetting unsaturated polyester resin, a filler, such as talc, and glass fibers, and additives including a hardening agent and a thickener, such as magnesium oxide. The SMC is supplied in sheets or blocks.
In compression-molding a SMC workpiece, such as a sheet or a block of a SMC, on a compression-molding press, the SMC workpiece is placed on a lower mold fixed on the bed of the compression-molding press, an upper mold is lowered by a hydraulic cylinder actuator to press the SMC workpiece between the upper mold and the lower mold to form the SMC workpiece in a,desired shape by thermosetting.
Since the SMC is a composite material consisting of the foregoing materials, the quality of the SMC differs with each lot. The characteristics of the SMC will vary with the additive content thereof and the viscosity of the SMC varies with time after the addition of the thickener to the SMC. Since the quality and fluidity of the SMC differs with each lot, it is possible that chips are formed in the molding due to the unsatisfactory flow of the SMC or flashes are formed due to excessive charging if molding conditions for compression-molding are fixed.
Various molding methods have been proposed to solve such problems. In compression-molding a resin in a cavity formed between upper and lower molds by a prior art compression-molding method, the hydraulic cylinder actuator of a hydraulic press is controlled in a speed control mode to control the pressing speed of the hydraulic cylinder actuator until pressure starts acting on the resin, the hydraulic cylinder actuator is controlled in a pressure control mode to control the pressing force of the hydraulic cylinder actuator after pressure has started acting on the resin, and the timing of changing the control mode from the speed control mode into the pressure control mode is controlled to solve the foregoing problems.
A compression-molding method disclosed in Japanese Patent Laid-open (Kokai) No. Hei 1-192500 detects the speed of the slide and controls the timing of changing the control mode on the basis of the speed of the slide. Compression molding methods disclosed in Japanese Patent Laid-open (Kokai) Nos. Hei 1-266996 and Hei 1-156018 detect the pressure in the cavity and controls the timing of changing the control mode on the basis of the pressure in the cavity. A compression-molding method disclosed in Japanese Patent Laid-open (Kokai) No. Hei 1-156019 detects the pressure in the pressure chamber of a hydraulic cylinder actuator and controls the timing of changing the control mode on the basis of the pressure in the pressure chamber of the hydraulic cylinder actuator. A compression-molding method disclosed in Japanese Patent Laid-open (Kokai) No. Hei 1-156023 detects the position of the slide and controls the timing of changing the control mode on the basis of the position of the slide.
These prior art compression-molding methods are effective when the difference in the contents of the components of the SMC between lots and the difference in the viscosity of the SMC between lots are relatively small and the charging rate varies slightly in the pressing speed control range. However, these prior art compression-molding methods are unable to deal with compression-molding a SMC having a very high viscosity or the quality of the SMC differs greatly with each lot, so that many nonconforming moldings are produced.